1. Field of the Invention
The present invention relates to a new method for preparing phosphorus-containing catalysts, to the polymerization catalysts that are made by this new method, and to the use of said catalysts for the polymerization of polyorganosiloxanes.
2. Description of the Prior Art
The desirability of polymerizing polyorganosiloxanes of low molecular weight to higher molecular weight fluids, gums and resins is well known in the art. The use of conventional catalysts, such as sulfuric acid and potassium hydroxide to facilitate the condensation reactions and the equilibration process that occurs during the polymerization of polyorganosiloxanes is also well known in the art and therefore needs no further elaboration.
When a known conventional catalyst is allowed to remain in the polymerized polyorganosiloxane, the high molecular weight polymer tends to undergo catalytic depolymerization in the presence of moisture or heat or both. Several methods for avoiding such catalytic depolymerization are known in the art and are directed to treatment of the catalyst.
One example of catalyst treatment using phosphonium compounds is the method described by U.S. Pat. No. 3,103,502 wherein an organopolysiloxane which has been polymerized with an alkali metal hydroxide catalyst such as potassium hydroxide is mixed with a quaternary phosphonium compound such as tetraethyl phosphonium iodide. A polyorganosiloxane gum, stable to heat can be obtained by such method. Such a method is better suited for fluid polymers than for the more viscous gum polymers and resin polymer, because intimate mixing of the polymer and the phosphonium compound is required and ease of mixing is facilitated by low viscosity.
An alternate method for avoiding catalytic depolymerization of organopolysiloxanes is taught by U.S. Pat. No. 2,883,366 wherein a basic quaternary phosphonium compound such as tetra-n-butyl phosphonium hydroxide and tetra-n-butyl phosphonium butoxide is used as the polymerization catalyst instead of an alkali metal hydroxide. When desired, the catalyst can be destroyed with heat without appreciably affecting the molecular weight of the polyorganosiloxane. However, quaternary phosphonium hydroxides and alkoxides are much more expensive than potassium hydroxide. In addition, said phosphonium hydroxides and alkoxides are much less stable than alkali metal hydroxides. A less expensive, more stable quaternary phosphonium catalyst has been sought for polymerizing siloxanes.